High strength hot dip galvanised steel strip

ABSTRACT

The invention relates to a high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements:
         0.10-0.21% C   1.75-2.50% Mn   0.04-0.60% Si   0.20-1.40% Al   0.001-0.025% P   0.0005-0.0050% B   max 0.50% Cr   max 0.20% Ti   max 0.004% Ca   max 0.015% N   the balance being Fe and inevitable impurities.

The invention relates to a high strength hot dip galvanised steel strip having improved formability, such as used in the automotive industry. The invention also relates to a method for producing such steel strip.

Such steel types are known and have been developed under the name of dual phase steel types. These steel types do not provide the formability as required in many applications for the automotive industry. For this reason, TRIP assisted dual phase steel types have been developed.

Formability, however, is not the only requirement for a TRIP assisted dual phase steel strip. The alloying elements should be low in amount to make the cost of the steel. as low as possible, and it should be as easy as possible to produce the steel strip at a is broader width both in the hot rolling mill and in the cold rolling mill. Moreover, the steel strip should be easy to coat with a zinc based coating, the steel strip has to have high strength and a good weldability, and should also exhibit a good surface quality. These requirements are especially important for industrially produced TRIP assisted dual phase steel types, which have to be formed into for instance automotive parts that will be spot welded into a body in White.

It is thus an object of the invention to find a composition of a high strength hot dip galvanised steel strip that strikes a balance between the formability, the homogeneity and the processability of the strip.

It is a further object of the invention to provide a high strength hot dip galvanised steel strip that has a good coatability during the hot dip galvanising process.

It is a still further Object of the invention to provide a high strength hot dip galvanised steel strip that has a good weldability.

It is another object of the invention to provide a high strength hot dip galvanised steel strip that has a good surface quality.

It is still another object of the invention to provide a high strength hot dip galvanised steel strip having a cost price that is as low as possible.

One or more of these objects are met according to the invention by providing a high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements:

0.10-0.21% C

1.75-2.50 % Mn

0.04-0.60% Si

0.20-1.40% Al

0.001-0.25% P

0.0005-0.0050% B

max 0.50 Cr

max 0.20% Ti

max 0.004% Ca

max 0.015% N

the balance being Fe and inevitable impurities.

The inventors have found that by a careful selection of the amounts of the main constituting elements of the steel, being carbon, manganese, silicon, aluminium, chromium and boron, a high strength hot dip galvanised steel strip can be produced that has the required formability, homogeneity, processability, strength and elongation, while at the same time providing a sufficient weldability, coatability and surface quality.

The inventors have especially found that it is advantageous to add boron to the composition of the steel. By adding boron, the hot rolled steel can be cooled fast enough on the run-out table to get a coiling temperature that provides a suitable microstructure for further processing. Moreover, due to the addition of boron, the inventors have found that the properties of the end product have a high degree of homogeneity. Thus, the steel strip can be produced in a width that is commercially interesting.

Boron suppresses ferrite formation during austenite cooling. This minimises local carbon enrichment in the strip. Therefore boron is normally avoided if TRIP type of steels are manufactured. The inventors have found that good TRIP assisted steel grades can be made by intercritically annealing of the cold rolled strip so that ferrite nucleation is not required.

Moreover, adding boron improves the hardenability of the steel, resulting in the possibility to use less of the other alloying elements. This results in an improved dimensional window for the steel strip, meaning a higher width to thickness ratio while the mechanical properties of the steel over the width of the strip remain suitable.

Boron segregates to the grain boundaries and replaces phosphorus at the grain boundaries, which makes it possible to have a higher P amount in the steel, while still achieving a good weldability.

The reason for the amounts of the main constituting elements is as follows, C: 0.10-0.21 mass %. Carbon has to be present in an amount that is high enough to ensure hardenability and the formation of martensite at the cooling rates available in a conventional annealing/galvanising line, Martensite is required to deliver adequate strength. Free carbon also enables stabilisation of austenite which delivers improved work hardening potential and good formability for the resulting strength level. A lower limit of 0,10 mass % is needed for these reasons. A maximum level of 0.21 mass % a has been found to be essential to ensure good weldability.

Mn: 1.75-2.50 mass %. Manganese is added to increase hardenability thus making the formation of hard phases like martensite or bainite easier within the cooling rate capability of a conventional continuous annealing/galvanising line. Manganese also contributes to the solid solution strengthening which increases the tensile strength and strengthens the ferrite phase, and also helps to stabilise retained austenite, Manganese lowers the transformation temperature range of the dual phase steel, thus lowering the required annealing temperature to levels that can be readily attained in a conventional continuous annealing/galvanising line. A lower limit of 1.75 mass % is needed for the above reasons. This lower limit is possible in view of the addition of other elements, such as boron. A maximum level of 2.50 mass % is imposed to ensure acceptable rolling forces in the hot mill and to ensure acceptable rolling forces in the cold mill by ensuring sufficient transformation of the dual phase steel to soft transformation products (fenite and pearlite). This maximum level is also given in view of the stronger segregation during casting and the forming of a band of martensite in the strip at higher values. Preferably, the amount of manganese is between 1.9 and 2.3 mass %, more preferably between 2.0 and 2.2 mass %.

Si: 0.04-0.60 mass %. Silicon provides solid solution strengthening thus enabling the attainment of high strength, and the stabilisation of austenite via strengthening of the ferrite matrix. Silicon very effectively retards the formation of carbides during overaging, thus keeping carbon in solution for stabilisation of austenite. For these reasons a lower limit of 0.04 mass % is needed. A maximum level of 0.60 mass % is imposed in view of the coatability of the steel strip, since high levels of silicon lead to unacceptable coating quality due to reduced adherence.

Al: 0.20-1.40 mass %. Aluminium is added to liquid steel for the purpose of de-oxidation. In the right quantity it also provides an acceleration of the bainite transformation, thus enabling bainite formation within the time constraints imposed by the annealing section of a conventional continuous annealing/galvanising line. Aluminium also retards the formation of carbides thus keeping carbon in solution, thus causing partitioning to austenite during overaging, and promoting the stabilisation of austenite. A lower level of 0.20 mass % is required for the above reasons. A maximum level of 1.40 mass % is imposed for castability, since high aluminium contents lead to poisoning of the casting mould slag and consequently an increase in mould slag viscosity, leading to incorrect heat transfer and lubrication during casting.

Cr: max 0.50 mass %. Chrome is added to increase hardenability. Chrome promotes formation of ferrite. A maximum level of 0.50 mass % is imposed to ensure that not too much martensite forms at the cost or retained austenite. It is also possible to add no chrome. Preferably, the amount of Cr is between 0.01 and 0.40 mass %, more preferably between 0.02 and 0.25 mass %.

Ti: max 0.20%. Titanium is mainly added to strengthen the steel, A maximum level of 0.20 is imposed to limit the cost of the steel. It is also possible to add no Ti,

Ca: max 0.004 mass %. The addition of calcium modifies the morphology of manganese sulphide inclusions. When calcium is added the inclusions get a globular rather than an elongated shape. Elongated inclusions, also called stringers, may act as planes of weakness along Which lamellar tearing and delamination fracture can occur. The avoidance of stringers is beneficial for forming processes of steel sheets which entail the expansion of holes or the stretching of flanges and promotes isotropic forming behaviour. Calcium treatment also prevents the formation of hard, angular, abrasive alumina inclusions in aluminium deoxidised steel types, forming instead calcium aluminate inclusions which are softer and globular at rolling temperatures, thereby improving the material's processing characteristics. In continuous casting machines, some inclusions occurring in molten steel have a tendency to block the nozzle, resulting in lost output and increased costs. Calcium treatment reduces the propensity for blockage by promoting the formation of low melting point species which will not clog the caster nozzles. It is also possible to add no calcium when the sulphur content is very low. Preferably, the amount of Ca is between 0.0005 and 0.003 mass %.

P: 0.001-0.025 mass %. Phosphorus interferes with the formation of carbides, and therefore some phosphorus in the steel is advantageous. However, phosphorus can make steel brittle upon welding, so the amount of phosphorus should he carefully controlled during steelmaking, especially in combination with other embrittling elements such as sulphur and nitrogen. On the other hand, in view of the addition of boron it is possible to have more phosphorus in the steel then usual.

The content of Nitrogen is limited to max 0.015 wt % as is typical for continuous casting plants. Usually, the amount of N is between 0.001 and 0.010 wt %.

In addition the reasons given above, the ranges for aluminium, boron, silicon, chromium and manganese are chosen such that a correct balance is found to deliver a transformation that is as homogeneous as possible on the run-out table and during coil cooling, to ensure a steel strip that can be cold rolled, and to provide a starting structure enabling rapid dissolution of carbon in the annealing line to promote hardenability and correct ferritic/bainitic transformation behaviour. Moreover, because aluminium accelerates and chromium decelerates the bainitic transformation, the right balance between aluminium and chromium has to be present to produce the right quantity of bainite within the timescales permitted by a conventional hot dip galvanising line with a restricted overage section. In practice, this means that the content of aluminium should be higher than the content of chromium.

According to a preferred embodiment, the amounts of Al and Si are chosen such that 0.60%<Al+Si<1.40%

According to another preferred embodiment, the amounts of Mn and Cr are chosen such that Mn+Cr>2.00%.

Preferably, the amounts of Al and Si are chosen such that Si≤Al.

Apart from the absolute contents of the elements as given above, also the relative amounts of certain elements are of importance.

Aluminium and silicon together should be maintained between 0.60 and 1.40 mass % to ensure suppression of carbides in the end product and stabilisation of a sufficient amount of austenite, with the correct composition, to provide a desirable extension of formability.

Manganese and chromium together should be above 2.00 mass % to ensure sufficient hardenability for formation of martensite and/or bainite and thus achievement of strength in a conventional continuous annealing line and hot dip galvanising line. In addition, Mn aids to stabilise retained austenite. Preferably, Mn+Cr should be above 2.10 mass %, especially when the amount of Si is low.

Al should preferably be present in an amount equal to or higher then Si in view of a good zinc coatability.

Preferably element C is present in an amount of 0.13-0.18%. In this range the hardenability of the steel is optimal while the weldability of the steel is enhanced, also by the presence of boron. More preferably element C is present in an amount of 0.14-0.17%. This amount of C has been found to work well in practice.

Preferably element Si is present in an amount of 0.05-0.50%, more preferably 0.05-0.40%. A amount of silicon lower then 0.50% improves the coatability of the steel strip, even more so when the amount of silicon is below 0.40%.

According to a preferred embodiment element Al is present in an amount of 0.30-1.20%, preferably an amount of 0.40-1.00%. A raised lower level of aluminium has the same effect as a higher amount of silicon, but hardly increases the strength of the steel. A lower upper limit of aluminium improves the castability of the steel.

The amount of element B is preferably between 0.0011 and 0.0040%, more preferably between 0.0013 and 0.0030%. to provide the desired hardenability and hence bring sufficient strength.

The amount of Ti is preferably max 0.10% so as to limit the cost of the steel and keep the dimensional window as large as possible. More preferably, the amount of Ti is between 0.005 and 0.05%.

Preferably the hot dip galvanised steel strip has an ultimate tensile strength Rill above 750 MPa and/or a 0.2% proof strength Rp of 430-700 MPa, preferably the difference between the middle and the edges of the steel strip being less then 75 MPa for both Rp and/or Rm, more preferably this difference being less then 60 MPa. These strength levels can be achieved with the composition according to the invention.

According to a preferred embodiment the hot dip galvanised steel strip has a microstructure, consisting of 20-50 volume % ferrite, 10-25 volume % retained austenite+martensite, of which 5-12% retained austenite, the remainder being tempered martensite, bainite and cementite.

According to a second aspect of the invention there is provided a method for producing a high strength hot dip galvanised steel strip as defined above, wherein the cast steel is hot rolled to a thickness of 2.0-4.0 mm and coiled at a Coiling Temperature CT below Bs−20° C. temperature and above Ms+60° C. temperature, the strip is cold rolled with a reduction of 40% or more, after which the strip is intercritically annealed at a temperature between Ac1 and Ac3 temperature, and the strip is averaged at a temperature below Bs temperature to form bainite and/or tempered martensite, after which the strip is hot dip galvanised.

Due to the coiling at a Coiling Temperature below Bs−20° C. temperature and above Ms+60° C. temperature, a well-defined microstructure is achieved, that can be cold rolled with the right reduction and annealed at the right temperatures and galvanised afterward, to get a galvanised steel strip with the right strength and the right properties.

Preferably the hot rolled coil has a microstructure consisting of 50-70 volume % ferrite, 20-50 volume % pearlite and/or bainite, and less then 10% cementite. With such a microstructure the coil has the right properties for further processing, especially for the annealing step, and can be cold rolled in a wide dimensional window.

According to a preferred embodiment the hot dip galvanised strip is tension rolled with a reduction of 0.2-0.8%. This percentage of tension rolling can provide the right mechanical properties to the strip, such as the tight strength level, while the other properties remain inside the desired window.

According to a further aspect of the invention there is provided a method for producing a high strength hot dip galvanised complex phase steel strip according to the first aspect of the invention, wherein the cast steel is hot rolled to a thickness of 2.0-4.0 mm and coiled at a Coiling Temperature CT below Bs−20° C. temperature and above Ms+60° C. temperature, the strip is cold rolled with a reduction of 40% or more, after which the strip is annealed at a temperature above Ad temperature plus 50° C., and the strip is overaged at a temperature below Bs temperature to form bainite and/or tempered martensite, after which the strip is hot dip galvanised. This complex phase steel strip can be made due to the precise Coiling Temperature and the prescribed annealing and overaging temperatures.

Preferably, this hot dip galvanised complex phase steel strip is tension rolled with a reduction of 0.4-2.0%, preferably with a reduction of 0.4-1.2%. This percentage of tension rolling can provide the right mechanical properties to the strip, such as the right strength level, while the other properties remain inside the desired window.

The invention will be elucidated hereafter.

FIG. 1 shows measurement of the ultimate tensile strength Rm and 0.2% proof strength Rp after annealing.

Line trials were performed, as shown in Table 1, indicated in milli-wt % unless indicated otherwise.

TABLE 1 Compositions of line trialled materials. Unless indicated different, the compositions are defined in mill-wt %. Bs and Ms values were calculated from¹. Al⁻ Cast C Mn P Si Cr B ppm zo Ti N ppm Al + Si Mn + Cr Bs −20° C. Ms +60° C. 1 151 2101 12 58 99 20 682 22 43 740 2200 581 477 2 161 2061 10 120 206 20 660 21 40 780 2267 573 472 3 147 2046 10 130 204 20 610 20 46 740 2250 578 479 4 149 2057 10 392 26 24 602 6 30 994 2083 583 476 5 148 2071 12 100 184 19 690 21 48 790 2255 578 478 6 153 2093 9 102 204 19 685 22 47 787 2297 573 475 ¹S. M. C. van Bohemen, Bainite and martensite Start Temperature calculated with exponential carbon dependence, Materials Science and Technology 28, 4 (2012) 487-495. Casts number 1, 2, 3, 5 and 6 were hot rolled with a hot roll finishing temperature of approximately 875° C. Cast number 4 was hot rolled with a hot roll finishing temperature of approximately 950° C.

TABLE 2 typical martensite + austenite distribution over different positions in the coil. Cast 6 Average martensite + austenite Head M 14.3 Head R 19.6 Middle M 18.9 Middle R 18.9 Tail M 17.7 Tail R 18.3

Typically the material was hot and subsequently cold rolled to a typical gauge in the range 0.8-2.0 mm.

For casts 5 and 6 microstructure and phase fractions defined is provided in FIG. 2. For cast 6 the microstructure over different sampling positions over the coil is provided in Table 2. The microstructure is given for the head, middle and tail of a coil. M indicates the middle of the strip, R the right hand side.

Using casts 1 to 4 and 6, a Dual Phase steel strip was produced. The hot roll finishing temperature was approximately 875° C. for all casts but for cast 4, as indicated above. The coiling temperature was between 500-520° C., well between Bs−20° C. and Ms+60° C., Subsequently, the material was cold rolled and intercritically annealed at around 800° C., and the overage temperature was 400° C. After hot dip galvanising, the strip was temper rolled with a reduction of around 0.3%,

Using cast 5, a Complex Phase steel strip was produced. The hot roll finishing temperature was approximately 875° C., the coiling temperature was 550° C. The material was cold rolled and intercritically annealed at around 840° C., and the overage temperature was 400° C. After hot dip galvanising, the strip was temper rolled with a reduction of 1.0%.

For cast 1 to 6, the mechanical properties were measured, depending on the coiling temperature. These mechanical properties are shown in Table 3.

TABLE 3 Tensile properties at mid coil and HEC measurements. VDA bendability Cast CT (° C.) Rp (MPa) Rm (MPa) Ag (%) A80 (%) n r HEC (%) @ 1 mm (L) 1 520 450 782 14.5 21.7 0.15 0.76 28-32* 137°* 1 498 490 817 14.0 21.0 0.14 0.54 2 506 505 808 13.5 20.4 0.13 0.69 18-30* 100-110°* 3 506 497 811 13.2 22.1 0.13 0.72 4 500 494 815 14.5 21.1 0.15 0.74 5 550 622 869 9.5 13.6 <0.1 0.70 33-36* 6 520 475 837 13.1 18.6 *typical values

Of the casts shown in Table 1, several slabs were produced. These slabs were hot rolled to strips having a thickness of 2.5-3 mm and thereafter the strips were coiled at different coiling temperatures (CT) between 500 and 590° C. These coils were cold rolled to a thickness of 1.3 mm, continuous annealed and hot dip galvanised.

Measurement of the ultimate tensile strength Rm and 0.2% proof strength Rp after annealing show that Rp and Rm increase when the CT is lower. This is shown in FIG. 1. The measurements also show that the elongation becomes slightly lower when CT is lower, but the elongation remains satisfactory at low CT.

FIG. 2 shows typical microstructures (Nital etched) obtained at the middle of the hot rolled strip product, after coiling and cooling down. Used is the composition of cast 1. For the left-hand picture, the coiling temperature CT was 500° C. The right-hand picture shows the same material but after a coiling temperature of 550° C. The dark phase is perlite/bainite and the light phase is ferrite; black dots are cementite. In the left-hand picture, perlite/bainite is present in 25-35%, ferrite 60-70% and cementite less than 10%. In the right-hand picture, perlitelbainite is present in 20-30%, ferrite 65-75% and cementite less than 10%.

Measurements have also shown that at low coiling temperatures the tensile properties over the width of strip are improved, meaning the difference between the middle of the strip and the edges of the strip are small. The difference is now at most approximately 50 MPa for Rp and Rm, whereas it used to be approximately 100 MPa.

FIG. 3 shows the variation in Rm and Rp over the width of the strip. FIG. 3a shows this variation for a strip composition that is not according to the invention, having a composition of 0.15 C, 2.05 Mn, 0.2 Cr, 0.7 Al, 0.07 Si, 0.015 Nb and 0.004 N (in wt %). The difference in Rrn between middle and edge of the strip is approximately 100 MPa, the difference in Rp is approximately 50 MPa.

FIG. 3b shows the variation in Rm and Rp for a strip with the composition of cast 1. This figure shows that it is possible to get a variation between the middle and the edge of the strip that is less than 20 MPa for both Rm and Rp. FIG. 3c shows in fact the same for a strip with the composition of cast 3. The strip shown in FIGS. 3b and 3c has been manufactured in accordance with the method of the invention.

FIG. 4 shows three different ways of graphically indicating the microstructures of the casts after using the method of the invention. These are the well known Picral, Nital and LePera representations. In the Picral graphs black represents bainite or tempered martensite. In the Nital graph the white spots indicate ferrite. In contrast, in the LePera graph white indicates (tempered) martensite+retained austenite. The differences between DP800 on the left-hand side and CP800 on the right-hand side are clearly visible.

The length indications in FIGS. 2 and 4 all indicate a length of 10 μm. 

1. A high strength hot dip galvanised steel strip consisting, in mass percent, of the following elements: 0.10-0.21% C 1.75-2.50% Mn 0.04-0.60 % Si 0.20-1.40% Al 0.001-0.025% P 0.0011-0.0040% B max 0.50% Cr max 0.20% Ti max 0.004% Ca max 0.015% N the balance being Fe and inevitable impurities.
 2. The steel strip according to claim 1, wherein the composition meets at least one feature selected from the group consisting of: amounts of Al and Si are such that 0.60%<Al+Si<1.40%, amounts of Mn and Cr are such that Mn+Cr>2.00%, and amounts of Al and Si are such that Si≤Al.
 3. The steel strip according to claim 1, wherein element C is present in an amount of 0.13-0.18%.
 4. The steel strip according to claim 1, wherein element Si is present in an amount of 0.05-0.50%.
 5. The steel strip according to claim 1, wherein element Al is present in an amount of 0.30-1.20%.
 6. The steel strip according to claim 1, wherein element B is present in an amount of 0.0015-0.0030%.
 7. The steel strip according to claim 1, wherein the element Ti is present in an amount of max 0.10%.
 8. The steel strip according to claim 1, wherein the hot dip galvanised steel strip has an ultimate tensile strength Rm above 750 MPa and/or a 0.2% proof strength Rp of 430-700 MPa.
 9. The steel strip according to claim 1, wherein microstructure of the hot dip galvanised steel strip consists of 20-50 volume % ferrite, 10-25 volume % martensite and retained austenite, of which 5-12 volume % retained austenite, the remainder being tempered martensite, bainite and cementite.
 10. The method for producing a high strength hot dip galvanised dual phase steel strip according to claim 1, wherein the cast steel is hot rolled to a thickness of 2.0-4.0 mm and coiled at a Coiling Temperature CT below Bs−20° C. temperature and above Ms+60° C. temperature, the strip is cold rolled with a reduction of 40% or more, after which the strip is intercritically annealed at a temperature between Ac1 and Ac3 temperature, and the annealed strip is overaged at a temperature below Bs temperature to form bainite and/or tempered martensite, after which the strip is hot dip galvanised.
 11. The method according to claim 10, wherein microstructure of the hot rolled coil consists of 50-70 volume % ferrite, 20-50 volume % pearlite and/or bainite, and less than 10% cementite.
 12. The method according to claim 10, wherein the hot dip galvanised strip is tension rolled with a reduction of 0.2-0.8%.
 13. The method for producing a high strength hot dip galvanised complex phase steel strip according to claim 1, wherein the cast steel is hot rolled to a thickness of 2.0-4.0 mm and coiled at a Coiling Temperature CT below Bs−20° C. temperature and above Ms+60° C. temperature, the strip is cold rolled with a reduction of 40% or more, after which the strip is annealed at a temperature above Ac1 temperature plus 50° C., and the strip is overaged at a temperature below Bs temperature to form bainite and/or tempered martensite, after which the strip is hot dip galvanised.
 14. The method according to claim 14, wherein the hot dip galvanised strip is tension rolled with a reduction of 0.4-2.0%.
 15. The steel strip according to claim 1, wherein element C is present in an amount of 0.14-0.17 %.
 16. The steel strip according to claim 1, wherein element Si is present in an amount of 0.05-0.40%.
 17. The steel strip according to claim 1, wherein element Al is present in an amount of 0.40-1.00%.
 18. The steel strip according to claim 1, wherein the element Ti is present in an amount of max between 0.005 and 0.05%.
 19. The steel strip according to claim 1, wherein the difference between the middle and the edges of the steel strip being less than 75 MPa for both Rp and/or Rm, more preferably this difference being less than 60 MPa.
 20. The method according to claim 14, wherein the hot dip galvanised strip is tension rolled with a reduction of 0.4-1.2%. 